A soft lockup is being detected in build_sched_domains() on 32 socket
Sapphire Rapids systems with 3840 processors.
topology_span_sane(), called by build_sched_domains(), checks that each
processor's non-NUMA scheduling domains are completely equal or
completely disjoint. If a non-NUMA scheduling domain partially overlaps
another, scheduling groups can break.
This series adds for_each_cpu_from() as a generic cpumask macro to
optimize topology_span_sane() by removing duplicate comparisons. The
total number of comparisons is reduced from N * (N - 1) to
N * (N - 1) / 2 (per non-NUMA scheduling domain level), decreasing the
boot time by approximately 20 seconds and preventing the soft lockup on
the mentioned systems.
RESEND because Valentin Schneider reported that PATCH 2/2 wasn't
delivered to all recipients.
Kyle Meyer (2):
cpumask: Add for_each_cpu_from()
sched/topology: Optimize topology_span_sane()
include/linux/cpumask.h | 10 ++++++++++
kernel/sched/topology.c | 6 ++----
2 files changed, 12 insertions(+), 4 deletions(-)
--
2.44.0
Optimize topology_span_sane() by removing duplicate comparisons.
The total number of comparisons is reduced from N * (N - 1) to
N * (N - 1) / 2 (per non-NUMA scheduling domain level).
Signed-off-by: Kyle Meyer <[email protected]>
Reviewed-by: Yury Norov <[email protected]>
---
kernel/sched/topology.c | 6 ++----
1 file changed, 2 insertions(+), 4 deletions(-)
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 99ea5986038c..b6bcafc09969 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -2347,7 +2347,7 @@ static struct sched_domain *build_sched_domain(struct sched_domain_topology_leve
static bool topology_span_sane(struct sched_domain_topology_level *tl,
const struct cpumask *cpu_map, int cpu)
{
- int i;
+ int i = cpu + 1;
/* NUMA levels are allowed to overlap */
if (tl->flags & SDTL_OVERLAP)
@@ -2359,9 +2359,7 @@ static bool topology_span_sane(struct sched_domain_topology_level *tl,
* breaking the sched_group lists - i.e. a later get_group() pass
* breaks the linking done for an earlier span.
*/
- for_each_cpu(i, cpu_map) {
- if (i == cpu)
- continue;
+ for_each_cpu_from(i, cpu_map) {
/*
* We should 'and' all those masks with 'cpu_map' to exactly
* match the topology we're about to build, but that can only
--
2.44.0
Add for_each_cpu_from() as a generic cpumask macro.
for_each_cpu_from() is the same as for_each_cpu(), except it starts at
@cpu instead of zero.
Signed-off-by: Kyle Meyer <[email protected]>
Acked-by: Yury Norov <[email protected]>
---
include/linux/cpumask.h | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index 1c29947db848..655211db38ff 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -368,6 +368,16 @@ unsigned int __pure cpumask_next_wrap(int n, const struct cpumask *mask, int sta
#define for_each_cpu_or(cpu, mask1, mask2) \
for_each_or_bit(cpu, cpumask_bits(mask1), cpumask_bits(mask2), small_cpumask_bits)
+/**
+ * for_each_cpu_from - iterate over every cpu present in @mask, starting at @cpu
+ * @cpu: the (optionally unsigned) integer iterator
+ * @mask: the cpumask pointer
+ *
+ * After the loop, cpu is >= nr_cpu_ids.
+ */
+#define for_each_cpu_from(cpu, mask) \
+ for_each_set_bit_from(cpu, cpumask_bits(mask), small_cpumask_bits)
+
/**
* cpumask_any_but - return a "random" in a cpumask, but not this one.
* @mask: the cpumask to search
--
2.44.0
On Tue, Apr 09, 2024 at 10:52:50AM -0500, Kyle Meyer wrote:
> Optimize topology_span_sane() by removing duplicate comparisons.
>
> The total number of comparisons is reduced from N * (N - 1) to
> N * (N - 1) / 2 (per non-NUMA scheduling domain level).
..
> - for_each_cpu(i, cpu_map) {
> - if (i == cpu)
> - continue;
> + for_each_cpu_from(i, cpu_map) {
Hmm... I'm not familiar with the for_each_cpu*(), but from the above
it seems only a single comparison? Or i.o.w. can i ever repeat the value?
And what about i < cpu cases?
--
With Best Regards,
Andy Shevchenko
On Tue, Apr 09, 2024 at 07:25:06PM +0300, Andy Shevchenko wrote:
> On Tue, Apr 09, 2024 at 10:52:50AM -0500, Kyle Meyer wrote:
> > Optimize topology_span_sane() by removing duplicate comparisons.
> >
> > The total number of comparisons is reduced from N * (N - 1) to
> > N * (N - 1) / 2 (per non-NUMA scheduling domain level).
>
> ...
>
> > - for_each_cpu(i, cpu_map) {
> > - if (i == cpu)
> > - continue;
> > + for_each_cpu_from(i, cpu_map) {
>
> Hmm... I'm not familiar with the for_each_cpu*(), but from the above
> it seems only a single comparison? Or i.o.w. can i ever repeat the value?
for_each_cpu() is a macro around for_each_set_bit() which iterates over each set
bit in a bitmap starting at zero.
for_each_cpu_from() is a macro around for_each_set_bit_from() which iterates
over each set bit in a bitmap starting at the specified bit.
The above (topology_span_sane()) currently does a "single comparison" for each
CPU in cpu_map, but it's called for each CPU in cpu_map and for each scheduling
domain level (please see build_sched_domains() in kernel/sched/topology.c).
> And what about i < cpu cases?
Those values have already been passed to topology_span_sane(). This patch uses
for_each_cpu_from() starting at cpu + 1 to prevent those duplicate comparisons.
Thanks,
Kyle Meyer
On Tue, 9 Apr 2024 at 17:54, Kyle Meyer <[email protected]> wrote:
>
> Add for_each_cpu_from() as a generic cpumask macro.
>
> for_each_cpu_from() is the same as for_each_cpu(), except it starts at
> @cpu instead of zero.
>
> Signed-off-by: Kyle Meyer <[email protected]>
> Acked-by: Yury Norov <[email protected]>
> ---
> include/linux/cpumask.h | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
> index 1c29947db848..655211db38ff 100644
> --- a/include/linux/cpumask.h
> +++ b/include/linux/cpumask.h
> @@ -368,6 +368,16 @@ unsigned int __pure cpumask_next_wrap(int n, const struct cpumask *mask, int sta
> #define for_each_cpu_or(cpu, mask1, mask2) \
> for_each_or_bit(cpu, cpumask_bits(mask1), cpumask_bits(mask2), small_cpumask_bits)
>
> +/**
> + * for_each_cpu_from - iterate over every cpu present in @mask, starting at @cpu
So I was confused why you were not using for_each_cpu_wrap while
reading the description which has the same comment :
"
* for_each_cpu_wrap - iterate over every cpu in a mask, starting at a
specified location
"
Could you clarify that it's not "every cpu present in @mask" but only
those after @cpu ?
> + * @cpu: the (optionally unsigned) integer iterator
> + * @mask: the cpumask pointer
> + *
> + * After the loop, cpu is >= nr_cpu_ids.
> + */
> +#define for_each_cpu_from(cpu, mask) \
> + for_each_set_bit_from(cpu, cpumask_bits(mask), small_cpumask_bits)
> +
> /**
> * cpumask_any_but - return a "random" in a cpumask, but not this one.
> * @mask: the cpumask to search
> --
> 2.44.0
>
On Tue, 9 Apr 2024 at 17:54, Kyle Meyer <[email protected]> wrote:
>
> Optimize topology_span_sane() by removing duplicate comparisons.
>
> The total number of comparisons is reduced from N * (N - 1) to
> N * (N - 1) / 2 (per non-NUMA scheduling domain level).
>
> Signed-off-by: Kyle Meyer <[email protected]>
> Reviewed-by: Yury Norov <[email protected]>
Acked-by: Vincent Guittot <[email protected]>
> ---
> kernel/sched/topology.c | 6 ++----
> 1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 99ea5986038c..b6bcafc09969 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -2347,7 +2347,7 @@ static struct sched_domain *build_sched_domain(struct sched_domain_topology_leve
> static bool topology_span_sane(struct sched_domain_topology_level *tl,
> const struct cpumask *cpu_map, int cpu)
> {
> - int i;
> + int i = cpu + 1;
>
> /* NUMA levels are allowed to overlap */
> if (tl->flags & SDTL_OVERLAP)
> @@ -2359,9 +2359,7 @@ static bool topology_span_sane(struct sched_domain_topology_level *tl,
> * breaking the sched_group lists - i.e. a later get_group() pass
> * breaks the linking done for an earlier span.
> */
> - for_each_cpu(i, cpu_map) {
> - if (i == cpu)
> - continue;
> + for_each_cpu_from(i, cpu_map) {
> /*
> * We should 'and' all those masks with 'cpu_map' to exactly
> * match the topology we're about to build, but that can only
> --
> 2.44.0
>
On Tue, Apr 09, 2024 at 10:52:48AM -0500, Kyle Meyer wrote:
> A soft lockup is being detected in build_sched_domains() on 32 socket
> Sapphire Rapids systems with 3840 processors.
>
> topology_span_sane(), called by build_sched_domains(), checks that each
> processor's non-NUMA scheduling domains are completely equal or
> completely disjoint. If a non-NUMA scheduling domain partially overlaps
> another, scheduling groups can break.
>
> This series adds for_each_cpu_from() as a generic cpumask macro to
> optimize topology_span_sane() by removing duplicate comparisons. The
> total number of comparisons is reduced from N * (N - 1) to
> N * (N - 1) / 2 (per non-NUMA scheduling domain level), decreasing the
> boot time by approximately 20 seconds and preventing the soft lockup on
> the mentioned systems.
>
> RESEND because Valentin Schneider reported that PATCH 2/2 wasn't
> delivered to all recipients.
Hmm... And this one doesn't have #1, at least for me. Can you please
resend again, so we all will have the series as a whole?
Also, can you rephrase this from patch #1 because it confuses people:
> + * for_each_cpu_from - iterate over every cpu present in @mask, starting at @cpu
Maybe: iterate over CPUs present in a @mask greater than a @cpu?
Or similar.
Thanks,
Yury
> Kyle Meyer (2):
> cpumask: Add for_each_cpu_from()
> sched/topology: Optimize topology_span_sane()
>
> include/linux/cpumask.h | 10 ++++++++++
> kernel/sched/topology.c | 6 ++----
> 2 files changed, 12 insertions(+), 4 deletions(-)
>
> --
> 2.44.0
On Tue, Apr 09, 2024 at 02:29:09PM -0500, Kyle Meyer wrote:
> On Tue, Apr 09, 2024 at 07:25:06PM +0300, Andy Shevchenko wrote:
> > On Tue, Apr 09, 2024 at 10:52:50AM -0500, Kyle Meyer wrote:
> > > Optimize topology_span_sane() by removing duplicate comparisons.
> > >
> > > The total number of comparisons is reduced from N * (N - 1) to
> > > N * (N - 1) / 2 (per non-NUMA scheduling domain level).
..
> > > - for_each_cpu(i, cpu_map) {
> > > - if (i == cpu)
> > > - continue;
> > > + for_each_cpu_from(i, cpu_map) {
> >
> > Hmm... I'm not familiar with the for_each_cpu*(), but from the above
> > it seems only a single comparison? Or i.o.w. can i ever repeat the value?
>
> for_each_cpu() is a macro around for_each_set_bit() which iterates over each set
> bit in a bitmap starting at zero.
>
> for_each_cpu_from() is a macro around for_each_set_bit_from() which iterates
> over each set bit in a bitmap starting at the specified bit.
>
> The above (topology_span_sane()) currently does a "single comparison" for each
> CPU in cpu_map, but it's called for each CPU in cpu_map and for each scheduling
> domain level (please see build_sched_domains() in kernel/sched/topology.c).
>
> > And what about i < cpu cases?
>
> Those values have already been passed to topology_span_sane(). This patch uses
> for_each_cpu_from() starting at cpu + 1 to prevent those duplicate comparisons.
So, it appears to me that commit message has a room to improve / elaborate based
on what you explained to me above.
Thanks!
--
With Best Regards,
Andy Shevchenko